1. Field of the Invention
The present invention relates to passive infrared motion detectors, occupancy sensors and similar devices, and more particularly to the infrared input section of these devices.
2. Description of the Related Art
Passive infrared motion detectors and occupancy sensors employ an array of Fresnel lenses covering an entrance aperture. This lens array is illuminated by thermal infrared radiation from the object of interest. For any particular angle of incidence each of the elements in the array of Fresnel lenses covering the entrance aperture generates a focal spot. The array of Fresnel lenses is designed so that as the object of interest moves across its field of view the system of focal spots moves across the sensitive area of a detector. The varying electrical output signal generated by the detector is processed to yield information about the state of motion of the object of interest.
Each element of the array of Fresnel lenses is designed to focus incident infrared radiation in a small angular range onto the sensitive area of a detector. The angular sectors, in which the elements of the array of Fresnel lenses focus onto one of the active areas of a detector, are interlaced by angular sectors which are not focused onto any sensitive area of any detector by any element of the array of Fresnel lenses. Moving infrared radiators are detected when they move from one angular sector across a boundary into an adjacent angular sector, leading to a rapid change in the amount of infrared power falling on the active area of a detector. Ordinarily all of the sectors are of the same angular size so that the maximum angle through which an object of interest can move without being detected, i.e. the angular resolution of the system, is equal to the angular size of one of these sectors. This assumes that the size and velocity of the radiating object and its distance from the entrance aperture are such that the infrared signal is greater than the minimum that can be detected by the system electronics.
One way to improve the angular resolution of the system is to increase the number of elements in the lens array. More specifically, the angular resolution of the system is approximately inversely proportional to the number of elements in the lens array. Thus, in order to achieve the smallest angular resolution, a lens array with as many elements as possible must be employed. On the other hand, the sensitivity and effective range of the system decrease if the size of the individual lenses of the array is decreased. The phrase “sensitivity of the system” refers to the size of the smallest radiating object that can be detected as a function of its distance from the detector. Thus, compromises must be made between the size of the entrance aperture, sensitivity, range and angular resolution of the system. For example, for any desired sensitivity and range there is a minimum size for each of the individual lenses of the array and hence a maximum number of elements for an entrance aperture of fixed size and a corresponding minimum angular resolution. The terms “focus” and “focusing” as used herein are intended to embrace any change in spot size and thus includes partially focusing and defocusing (e.g. dispersing energy).